Connectivity of the American Agricultural Landscape: Assessing the National Risk of Crop Pest and Disease Spread

T he United States contains one of the most important crop production areas in the world.According to the most recent national agricultural census, 1.8 billion bushels of wheat, 10.5 billion bushels of maize, and a wide range of other crops were produced in 2006 from 126 million hectares (315 million acres) in the conterminous United States (USDA NASS 2007).However, owing to the concentrated nature of the agricultural landscape and limited genetic diversity of many crops (Parker 2002, Harrington 2003), crop production is vulnerable to disease and damage by insect pests.Farm legislation that provides subsidies to growers for only a small number of crop species may inadvertently contribute to this homogeneity (e.g., Biermacher et al. 2006).Meanwhile, an average of 10 new crop pests are estimated to enter the United States accidentally each year, usually through shipments of plant materials, produce, or packing materials from other continents through US ports (Work et al. 2005).The economic damage caused by the spread of exotic crop pests is significant.The US Department of Agriculture (USDA) and other US government agencies spend more than $1 billion annu-ally (Parker 2002) in research, risk assessment, and emergency response to outbreaks, and in public education, outreach, and extension.Government agencies in the United States have begun to assess food security issues (Parker 2002), and organizations concerned with agricultural emergency response, such as the USDA Animal and Plant Health Inspection Service (APHIS), have procedures in place that target prevention, response, and recovery from a crop biosecurity breach (USDA and USDOI 2005).Geospatial analytical tools, such as the North Carolina State University/APHIS Plant Pest Forecasting System (NAPPFAST; Magarey et al. 2007) and CLIMEX (Sutherst et al. 1999), have been applied to forecast the risk that pathogens and pests pose to agriculture as a result of climatic conditions.Additional geospatial tools that incorporate models of pathogen and pest dispersal are still needed, both to anticipate and react to new outbreaks and to evaluate risk and form priorities for management of ongoing problems.However, tool and model development are hampered by the complexity of interactions among host, pest or pathogen, and